You maybe limited on how much RAM your system will support. You’ll need to review the owners guide for your system (logic board). in addition you’ll need to review your I/O interfaces to see what you have. If you have a IDE (ATA) interface your options are limited, hopefully you have SATA interfaces. Next you’ll need to see if you have any free ports to add the drive Vs swapping it out which will be harder. And lastly, are you up to swapping things around as well as installing a fresh copy of your OS.